1 IEEE 802.3bn Orlando, FL March 18-21, 2013 IEEE 802.3bn Orlando, FL March 18-21, 2013 1
PHY Link Channel
Avi Kliger, Broadcom
Nicola Varanese, Qualcomm
2 IEEE 802.3bn Orlando, FL March 18-21, 2013
Downstream:
PLC broadcasts information necessary to enable the operation of the PHY layer (e.g., proper demodulation and decoding of data)
PLC conveys CNU-specific information (supported MCS in US, power control commands, timing advance commands)
Upstream:
PLC collects CNU-specific information (e.g., supported MCS in DS)
PLC transmission allows estimation of supported MCS in US (sounding), power control, timing advance commands
In order to ensure scalability, each PHY channel (192MHz) has its own, dedicated PLC
The standard will not mandate to place PLC in a specific frequency location (e.g., around DC subcarrier)
PLC does not convey MAC Control information (GATE/REPORT messages)
Definition of PHY Link Channel (PLC)
3 IEEE 802.3bn Orlando, FL March 18-21, 2013
Must be detectable by any new CNU trying to join the network
Center frequency of PLC is not necessarily the same as center frequency of the corresponding OFDM Channel
CNU doesn’t need any information on OFDM channel frequency usage, FFT size and CP size, except for a raster of the PLC center frequencies
PLC is transparent to upper layers
No additional buffering requirements
No additional jitter and latency
Requirements for PHY Link Channel
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Downstream PLC
5 IEEE 802.3bn Orlando, FL March 18-21, 2013
Purpose of PLC
Example to information carried by the PLC Data:
OFDM channel ID
DS Bandwidth (Number of available sub-carriers in this OFDM channel)
Exclusion bands, e.g.,
– Start frequency of each EB
– Stop frequency of each EB
DS Continuous Pilots locations
Information regarding PHY frame structure
– TDD split in terms of US/DS OFDM symbols and guard interval
– FDD US information: carrier frequency, number of available subcarriers
Frequency Interleaving pattern (logical to physical subcarrier mapping)
Time Interleaving depth for DS
Active profiles (active MCS’s)
Timing advance information for specific CNU(s)
Assignment of specific CNU(s) to a given DS/US profile
Power control information for specific CNU(s)
Message
for a specific CNU
Broadcast
Message
6 IEEE 802.3bn Orlando, FL March 18-21, 2013
QAM16 constellation for data
~12 dB more robustness to AWGN than QAM256 to protect against bad SNR with a simple error correction code
Forward Error Correction code
Provides more robustness against channel notches and noise bursts
Short code to reduce latency and complexity
Center frequency is located at one of today’s DOCSIS center frequencies (as determined by EIA or other channel plan in use by operator)
PLC Structure /1
PHY Link
Center Freq
PHY Link Channel
(0.4-1.6 MHz)
OFDM sub-carriers
Downstream OFDM channel (up to 192 MHz)
Channel
Center Freq
frequency
> 6MHz
7 IEEE 802.3bn Orlando, FL March 18-21, 2013
Uses dedicated subcarriers
8, 16, 32 subcarriers for 4k FFT size
16, 32, 64 subcarriers for 8k FFT size
Reducing the number of subcarriers for PLC Data
Time interleaving gains against burst noise (FEC codeword is spread over multiple symbols)
Small overhead even if very few resources are available
Increasing the number of subcarriers for PLC Data
Frequency interleaving gains against channel notches
Negligible overhead if available bandwidth is larger than 24MHz
PLC includes Start-of-Frame Delimiter (SFD) for FEC word alignment
PLC SFD and Data are repeated every PHY frame (corresponds to pilot repetition period)
PLC Structure /2
Time
Freq
uenc
y Data SFD
X subcarriers
1 PHY frame
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Sent only on the subcarriers dedicated to the PLC
Enables alignment to PLC FEC codeword and aids the detection of the PLC
Consists of a BPSK PN sequence in frequency
Eight symbol SFD gives excellent robustness at bad SNR
SFD is repeated every 128 (4k FFT) or 64 (8k FFT) OFDM symbols for FDD, 256 (4k FFT) or 128 (8k FFT) OFDM symbols for TDD
This gives a SFD interval of ~2.75 mSec with CP=1.5uS in the FDD case
The spec could allow a larger repetition period if necessary
Trade-off between resources employed for SFD and PLC data throughput
For FDD, it could be aligned with rotating pilot cycle so that pilots do not interrupt SFD
Start-of-Frame Delimiter (SFD)
SFD Data SFD Data
X symbols
128/64 symbols (~ 2.75- mSec) 128/64 symbols
9 IEEE 802.3bn Orlando, FL March 18-21, 2013
Consider Forward Error Correction code to improve robustness
Trade code performance (code rate) with latency to determine FEC
Is robustness against burst noise is required?
A burst event will only affect one to two OFDM symbols
– FEC can readily be designed to correct for this
Ingress, narrowband notches, etc.:
Ingress is often predictable; locate channel where ingressors are not expected
– CLT is capable of moving the PLC channel if required
For narrowband notches (possibly seen by certain modems due to local micro-reflections), coded QAM 16 constellation gives an additional 12 dB or more robustness (compared to QAM 256 or higher)
With multiple OFDM channels use a PLC on each channel
PLC Robustness Against Burst Events and Ingress
Preamble PLC Data (repeated)
Downstream MAC/user data
Downstream MAC/user dataDownstream MAC/user data
128 symbols 128 symbols
f1
f2
Preamble PLC Data
Increase robustness
Expedite detection
If this is not adequate, PLC can be duplicated at two different frequencies
Or, the channel could alternate between two different frequencies, with the same information being sent on both
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Regular pilot symbols (this example shows the TDD configuration)
Continual pilot symbols:
Present on every OFDM symbol (just like PLC)
E.g., with 50kHz spacing, continual pilot symbols occur every 128 subcarriers (i.e., at the borders of legacy 6MHz channels)
Not transmitted within exclusion bands
Additional continual pilots can be used as edge pilots at the borders of each exclusion band (configured via PLC)
Used to track channel variations (e.g., phase tracking), improve channel estimate and locate the PLC (searcher algorithm)
Proposed PHY Frame Structure
193192191
6968676665646362616059
76543210-1-2-3-4-5-6-7
-59-60-61-62-63-64-65-66-67-68-69
-191-192-193
Time
Fre
qu
en
cy
DC subcarrier
Re
gula
r p
ilot
Co
nti
nu
al P
ilot
Co
mb
ine
d P
ilot
Frame idx 0 1
Symbol idx 0 1 2 3 4 5 6 7 n-2 n-1 0 1 2 3 4 5 6 7
Subframe idx 0 1 2 3 (n-2)/2 0 1 2 3
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Physical layer frame
1 OFDM symbol
Physical layer frame Physical layer frame
freq
uen
cyG
uaran
teed b
and
time
PLC
ContinualPilots
The standard will define the notion of minimum guaranteed continuous band
No exclusion bands and nulled subcarriers are allowed within at least a portion of the channel as wide as this minimum band
Possible values: 6MHz, 12MHz, 24 MHz
PLC is placed at the center of such a band
Additional continual pilots are placed within such a band, symmetrically with respect to the PLC (see figure)
Searcher algorithm is based on the continual pilots related to the PLC
Resources Reserved for PLC
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TDD cycle
1 OFDM symbol
TDD cycle TDD cycle
freq
uen
cyG
uaran
teed b
and
time
PLC
ContinualPilots
UPSTREAM UPSTREAMUPSTREAM
PHY frame
SFD and PLC data repeated every 256 (128) OFDM symbols (TDD DS PHY frame duration)
PLC provides information on
TDD cycle duration and DS/US split (i.e., DS and US time-slot duration)
TDD guard interval duration
SFD located at the start of a DS time-slot
As for FDD, PLC enables PHY frame synchronization and alignment to the TDD DS/US cycle
TDD-specific Aspects
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Scan designated channel plan (6 MHz or 8 MHz) looking for continual pilots and/or PLC SFD
The sequence below is an example and is implementation dependent
1. Find FFT size and CP size using correlation
2. Find FFT boundaries
3. Find fractional frequency offset
4. Find continual pilots (and integer frequency offset)
5. Find SFD (Preamble)
6. Estimate channel using SFD
All should be accomplished in a single Preamble period on the average
Begin receiving PLC
Decode PLC to find messages describing OFDM channel parameters (center frequency, available sub-carriers, FEC/Interleaving pointers, profile, pilots …)
Start Admission process and Ranging
Begin receiving Data
Initial Acquisition Sequence
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Detection of the Downstream PLC –1
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Searcher outcome:
OFDM symbol boundary estimate
Carrier Frequency Offset estimate
Carrier Frequency Offset (CFO)
Fractional CFO
Integer CFO (alignment to PLC subcarriers)
Searcher operation:
Searcher processes 12 consecutive symbols
Time domain processing
– Perform OFDM symbol boundary search
– Perform fractional CFO estimation and correction
Frequency domain processing
– Search for continual pilots (integer CFO estimation)
– Re-estimate fractional CFO
Simulation Setup /1
𝐶𝐹𝑂 = 𝑘∆𝑆𝐶 + 𝛿
16 IEEE 802.3bn Orlando, FL March 18-21, 2013
Minimum guaranteed continuous band of 25.6MHz
4k FFT size (50kHz subcarrier spacing)
512 subcarriers
4 continual pilots available for acquisition (1 every 128 subcarriers)
+4.76dB pilot boost
ReDeSign Channel Model 2
Unrealistic according to some: >12dB dynamic range within 6MHz
Sampling frequency offset: 80ppm
Integer CFO 𝑘 : 10
Fractional CFO 𝛿 : 0.26 x 50kHz
Total CFO : 513kHz
Simulation Setup /2
17 IEEE 802.3bn Orlando, FL March 18-21, 2013
Simulation Results /1
Fractional CFO estimation
Time domain
– CP-based fractional CFO estimation
Frequency domain
– Pilot-based fractional CFO estimation (refinement of CP-based estimate):
Successive
refinement
-10 -5 0 5 10 15 10
-3
10 -2
10 -1
10 0
SNR [dB]
RM
SE
[subcarr
ier
spacin
g]
CP based
Pilot based
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Integer CFO estimation (subcarrier index alignment)
Frequency domain
– Search for continual pilots (integer CFO estimation)
Success probability:
Simulation Results /2
-10 -5 0 5 10 15 0
10
20
30
40
50
60
70
80
90
100
SNR [dB]
Lockin
g p
robabili
ty [
%]
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Detection of the Downstream PLC – 2
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PLC receiver tasks at Initialization
Wideband processing and decimation
Divide the full-band into M 6 MHz signals around Center frequency raster
Look for FFT size/ CP size among hypothesis crossing a threshold
Recover timing and frequency
Find FFT boundary
Using best CP size look for Preamble
Preamble search can done serially or in parallel on multi 6 MHz bands
Trade complexity and acquisition time
Preamble detector a very simple 8-tap 1-bit Correlator
For fast detection use M such Correlators
Wideband
Processing/
Filtering
Decimate to 6 MHz band
FFT Size/ CP size/
Frequency Acquisition
Look for Preamble /
Correlation
Receive
PLC Data
Wideband
operation
6 MHz
0.4-1.6
MHz
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Preamble Acquisition Time
Preamble every 64/128 symbols (2.75 mSec)
Scan 150 frequencies
CP / FFT detection takes
64 uSec to detect the CP/FFT
10 hypothesis done serially < 25 mSec
Search time depends on OFDM band
Serial Preamble detection
Worst case Preamble detection time ~ 0.5 Sec (150*2.75)
Parallel Preamble detection (e.g. M=32)
Worst case Preamble detection time ~ 15.5 mSec
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CP size, Frequency and PLC detection scheme
CP detection
Input signal - X(n) Wideband
Processing
6 MHz
IFFT 128
6 MHz
IFFT 128
M , 6 MHz channels
Remove CP
FFT 128
Cross-Correlation &
CFO detection
CP size,
symbol
boundary
Frequency
Correction
fractional
frequency
offset
Chose hypothesis
and correspondent
frequency offset
23 IEEE 802.3bn Orlando, FL March 18-21, 2013
192 MHz band with 24 MHz OFDM and rest is single carrier signals
CP resolution is 32/204.8 = 0.16 uSec
SNR = 15 dB
Carrier and sampling frequency offsets: 170 ppm
Actual CP sizes of 0.625 uSec and 1.09 uSec
FFT Size = 4K
TX window included
0 50 100 150 200 250-16
-14
-12
-10
-8
-6
-4
-2
0
OFDM
Single
Carrier
24 MHz
0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
50
100
150
200
250
300
350CP size peak correlation at 6.4 MHz output filters
CP size (uSec)
peak c
orr
ela
tion (
linear)
CP size 224/204.8 uSec
CP size 128/204.8 uSec
0 100 200 300 400 500 600 700 8000
50
100
150
200
250
300
350CP size peak correlation frequency response, SNR=15 dB, OFDM channel 24 MHz
frequency (MHz
peak c
orr
ela
tion (
linear)
CP Size Recovery with Legacy Services /1
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0 50 100 150 200 250-18
-16
-14
-12
-10
-8
-6
-4
-2
0
0 100 200 300 400 500 600 700 8000
50
100
150
200
250
frequency (MHz
peak c
orr
ela
tion (
linear)
CP size peak correlation frequency response, SNR=15 dB, OFDM channel 24 MHz
CP size 224/204.8 uSec
CP size 128/204.8 uSec
0.5 1 1.5 2 2.5 3 3.5 4 4.5 540
60
80
100
120
140
160
180
200
220
240CP size peak correlation at 6.4 MHz output filters
CP size (uSec)
peak c
orr
ela
tion (
linear)
192 MHz band with 24 MHz OFDM and rest is single carrier signals
CP resolution is 32/204.8 = 0.16 uSec
SNR = 15 dB
Carrier and sampling frequency offsets: 170 ppm
Actual CP sizes of 0.625 uSec and 1.09 uSec
FFT Size = 4K
TX window included
CP Size Recovery with Legacy Services /2
25 IEEE 802.3bn Orlando, FL March 18-21, 2013
Cross correlation and CFO detection
N = Preamble length (8,16,32)
Correlator has N multipliers by 1 bit
The Small FFT and Correlator blocks allow to use parallel data to expedite acquisition time
N
IFFT
D
X
()*
SD
EC
ISIO
N
N samples
D DNumber of preamble repetitions-1
C1(n)
X
Known sequence
X
Known sequence
X
Known sequence
N
IFFT
N
IFFT
D
X
()*
SN samples
D DNumber of preamble repetitions-1
C1(n)
D
X
()*
SN samples
D DNumber of preamble repetitions-1
C1(n)
26 IEEE 802.3bn Orlando, FL March 18-21, 2013
SFD detection performance
SFD is a repetition of a PN sequence in the frequency domain
Detection probability vs. SNR with different numbers of symbols and sub-carriers were simulated
Results depicted in table below
SFD Detection Performance
Number of symbols with 99.9% Preambles detection
SNR = 10
dB
SNR = 15
dB
SNR = 25
dB
32 4 4 4
16 6 4 4
8 8 6 4
Numer of symbols Num of
sub
carriers
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Final Proposal
28 IEEE 802.3bn Orlando, FL March 18-21, 2013
Proposal for PLC
In order to ensure scalability, each PHY channel (192MHz) has its own, dedicated PLC
The standard will not mandate to place PLC in a specific frequency location (e.g., around DC subcarrier)
PLC does not convey MAC Control information (GATE/REPORT messages)
PLC is transparent to upper layers: no additional jitter
PLC does not entail additional buffering requirements
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